This invention relates to marine seismic cables, and, more particularly, to a towed seismic cable which flexible solid materials, disposed within the cable, buoyantly support.
For many years, the marine seismic exploration industry has relied on fluid-filled seismic cables. Marine seismic cables are one of the most critical components used with seismic exploration vessels, and have a direct affect on the accuracy of the results which researchers and technicians obtain. The amount and type of fluid in oil-filled cables must be adjusted dependent on changes in water temperature and/or salinity. Also, oil-filled cables are prone to leakage. The outer jacket of the seismic cables of the prior art is prone to rupture or tearing. This is particularly undesirable because this exposes the internal electrical components to seawater, and disrupts the buoyancy of the cable.
The prior art has used communication coils to communicate with various external devices, such as cable-levelers. A cable-leveler attaches to the outside of the cable in order to control the depth of the cable, according to magnetic induction signals which the cable-leveler receives from the communication coil inside the cable. Other external devices such as location modules, compasses, and depth transducers may also use the communication coil for telemetry. This wireless communication system, commonly known in the art, eliminates the need for a physical connection between the communication coil inside the cable and the external device, thus better ensuring water-tight enclosure of the cable. The prior art wireless communication system used with a kerosene-filled cable, or non-solid cable, includes a single coil that is attached with plastic "ties" to the bundles of wires which make up a center core of such cable.
U.S. Pat. Nos. 5,089,668, 5,141,796, and 5,471,436 disclose the use of a buoyant material to permit solid or semi-solid composition of a seismic cable. There is no way to attach the existing communication coil to the center core of a solid cable without the center core, and its attached communication coil, moving with respect to the external device. A center core occupies the center of a solid cable. The center core includes a rope centered within electrical cables, all within a sheath. Therefore, the communication coil must be placed adjacent to the center core. The communication path between the communication coil and a receiver or a transmitter in the external device is a straight line. If the cable twists in the water so that the center core obstructs all or part of the communication path, then the magnetic induction between the external device and the communication coil is disrupted or broken, both by distance and by the shielding caused by the center core. This has an adverse effect on the ability of the communication coil to communicate with the receiver or transmitter in the external device.
The industry needs a solid seismic cable which enables attachment of external devices to the cable without concern for the radial alignment of the device with a communication coil inside the cable.